Highlight failing jobs on the dashboard

> When we have jobs that have over 3% failure rates we should highlight
> those so that peoples attention is drawn to deal with the failure.
>
> They would then go to the job view to see what the details are where
> they could filter by failure, but that's a different story...
>
> This is just about calculating and highlighting those that need their
> attention.

— https://www.pivotaltracker.com/story/show/121206123

This commit:

- calculates the failure rate for each job
- makes jobs with a failure rate of > 3% go red on the dashboard

app/statistics_utils.py

@@ -69,3 +69,21 @@ def statistics_by_state(statistics):
             'failed': statistics['emails_failed']
         }
     }
+
+
+def get_failure_rate_for_job(job):
+    if not job.get('notifications_delivered'):
+        if job.get('notifications_failed'):
+            return 1
+        return 0
+    return (
+        job.get('notifications_failed', 0) /
+        (job.get('notifications_failed', 0) + job.get('notifications_delivered', 0))
+    )
+
+
+def add_rate_to_jobs(jobs):
+    return [dict(
+        **job,
+        failure_rate=(get_failure_rate_for_job(job)) * 100
+    ) for job in jobs]